MiB vs. MB: Understanding the Units for Data Rates and Bandwidth#

Are you confused about the units MiB or MB (MiB/s or MB/s)?

If yes, that's not a surprise, since there are several units or prefixes available which are often used incorrectly. This article describes the units.

kilo, Mega, Giga or Tera are prefixes meant for and based on the decimal metric system. They stand for multiples of 103 (1000) and are standardized in the International System of Units (SI) (see first table below).

However, bits and bytes aren't covered by the SI and also not based on the decimal system. They're binary instead.

A while ago, when computers became popular, professionals started to use the prefixes also for the binary system but with a different meaning (see second table below).

Today, most manufacturers use megabyte (MB) to mean 1 048 576 bytes, but the manufacturers of computer storage devices usually use the term to mean 1 000 000 bytes. Some designers of local area networks have used Megabit per second (Mbps / Mbit/s) to mean 1 048 576 bit/s, but all telecommunications engineers use it to mean 106 bit/s. And if two definitions of the Megabyte are not enough, a third Megabyte of 1 024 000 bytes is used to format the familiar 90 mm (3 1/2 inch), "1.44 MB" floppy disk.

This causes confusion and a high potential for incompatibility in standards and in implemented systems.

With the aim of avoiding the above-mentioned ambiguity, the International Electrotechnical Commission (IEC) adopted new binary prefixes in 1998. Each binary prefix is formed from the first syllable of the decimal prefix with the similar value, and the syllable "bi". The symbols are the decimal symbol, always capitalized, followed by the letter "i". That is, e.g., Mebibyte or MiB.

According to these standards, kilo, Mega, Giga, Tera etc., would only be used in the decimal sense, even when referring to data storage capacities: kB and MB would denote one thousand and one million bytes respectively (consistent with the metric system). While new terms such as kibibyte, Mebibyte and Gibibyte, with symbols KiB, MiB and GiB, would denote 210 (1024), 220 and 230 bytes respectively.

You know now what those prefixes mean. However, you never know if a company or user is using a prefix correctly. So when in doubt, double-check.

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